346 



THE BIOLOGY OF MARINE ANIMALS 



GRAVITY SENSE 



Many organisms orient towards or away from the source of the force of 

 gravity by a form of response termed geotaxis. The specialized gravity 

 receptor for geotaxis is the statocyst. This is a fluid-filled chamber lined 

 by a sensory epithelium bearing hair cells, and containing a solid or semi- 

 solid body known as a statolith (Fig. 8.29). The latter rests on part of the 

 epithelium, or hangs from the wall, and mechanically stimulates the sense 

 cells. Any displacement of the statocyst from its resting position, owing to 

 movement of the animal, alters the pattern and force of stimulation and 

 leads to appropriate adjusting reactions. These cease when the statolith 

 has returned to its original position. 



A beautiful experiment demonstrating statocyst functioning was per- 

 formed by Kreidl over half a century ago. In shrimps the statoliths are lost 



Fig. 8.29. Section through the Statocysts of Pecten inflexus 



l.St., r.St., left and right statocysts; A, external duct; N, nerve; h.B. and r.B., connec- 

 tive tissue. (After von Buddenbrock.) 



at each moult and are replaced anew with sand grains from the environ- 

 ment. When iron dust was provided, it became incorporated in the stato- 

 cysts in place of sand particles. These shrimps now became sensitive to the 

 pull of a magnet: when, for example, a magnet was held above the animal, 

 the pull on the statoliths towards the top of the statocyst caused the 

 animal to turn over on its back. 



Statocysts found in scyphomedusae and ctenophores are concerned 

 with orientation reflexes. Medusae have eight statocysts symmetrically 

 arranged around the margin. When the animal is tilted, stimulation of the 

 statocysts causes the musculature of the lowered portion of the umbrella to 

 contract more strongly than the upper portion and the animal soon rights 

 itself. Extirpation of several neighbouring statocysts causes permanent 

 disorientation. Ctenophores have a single apical statocyst which has 

 nervous connexions with the comb rows. The animals are negatively or 



